Two Million
Pixels are Better than 1 Million Pixels:

Executive summary

The argument that
1080x1920, 30 frames/sec interlaced (1080I) will give better picture quality than
720x1280, 60 frames/sec progressive (720P) because it has twice as many picture
elements (pixels) per frame is the most recent erroneous idea put forth by those
who have been pushing the NHK 1125/60 system for years as a world-wide production
standard. The earlier arguments all turned out to be incorrect; this new one is
but the latest attempt to foist off this obsolete technology on the American broadcasting
industry. In fact, the vertical resolution actually achieved in 1080I is lower
han that actually achieved in 720P, while the horizontal resolution is considerably
less than 1920 pixels, as clearly shown by objective tests carried out at ATTC.
Subjective tests carried out by ATEL showed that the perceived picture quality
of the two systems was comparable.

Background

To understand the
current controversy, it helps to recall a little history. Interlace, with its
many problems, has always been used in television. Nevertheless, current systems
-- NTSC and PAL -- for all their defects, have been very successful commercially.
About 1970, NHK embarked upon a project to develop the next generation of TV
systems.
They had in mind a system much like NTSC but with a wider aspect ratio, about
twice the vertical and horizontal resolution, and about five times the bandwidth.
It was intended to be the "FM" of video, delivered by satellite, while NTSC, delivered
terrestrially, was to be the "AM." The NHK system used interlace because it was
a straightforward scale-up of existing analog systems, and it did not provide
for separate production and distribution formats.

System design
was done by NHK and equipment development by Sony, Matsushita, Toshiba, and
other large electronics manufacturers. In the late seventies, the system was
successfully demonstrated, but the special wide-band transponders were evidently
deemed uneconomical. By 1984, NHK had developed a bandwidth-compression system
called MUSE to permit transmission in standard transponder channels as used
for NTSC. It was at this point that the concept of using the original 30-mhz
system as a production system was first enunciated. Eventually, a full line
of production equipment for the 1125-line system was developed in Japan. With
the help of SMPTE, an attempt was made to have ANSI recognize the NHK system
as a production standard. ANSI at first agreed, but reversed itself on appeal
by ABC on the grounds that the system was not actually being widely used for
the proposed application.

Ever since, there
has been unremitting pressure to use this system as a production standard, even
to the extent of enlisting the help of the State Department. It was decisively
turned down by the EBU, and that had seemed the end of the matter until the
formation of the Grand Alliance.

As everyone knows,
the FCC Inquiry set up in 1987 to develop HDTV transmission standards was turned
on its head in 1989 by the proposal from General Instrument for an all-digital
system. In the first round of tests at ATTC, there were four digital systems
(two progressive and two interlaced), plus MUSE and ACTV, an NTSC-compatible
analog system from the Sarnoff Laboratories. MUSE placed last in performance
and it and the other remaining analog system were withdrawn. The four remaining
digital system proponents were forced into a shotgun wedding by ACATS, forming
the Grand Alliance. Evidently no system proponent was willing to give up his
format, so all were included. (The standard-definition formats were added later
by ATSC, with no testing at all.) Curiously, the interlaced systems, which had
used 960x1408 and 960x1440 in the initial tests, when combined into one, were
raised to 1080x1920, thus reviving the NHK system as the natural production
standard and providing a potential market for the production equipment already
developed by Japanese companies. (The production equipment now being offered
for sale is actually 1035I, not 1080I.)

The interlace
arguments pro and con were spelled out in 1996 in submissions to the FCC by
the interested parties as the Commission was considering the standard proposed
by ACATS in 1995. Foremost among the interlace advocates were Sony, ATSC, the
Grand Alliance, and North American Philips. It appears that the ATSC and GA
submissions were both to a large extent written by Robert Graves, who was hired
by the GA to get the proposal accepted and who is now head of ATSC. The main
reasons advanced at that time for using interlace were:

Interlace doubles
the vertical resolution for a given bandwidth and frame rate.

P requires
more bandwidth or channel capacity than I for the same resolution.

We have to
have interlace so that we can have cheaper receivers.

P raises costs
for broadcasters.

No one knows
how to make P cameras with adequate SNR.

Many programs
that will be used for SDTV transmission already exist in NTSC format.

Every one of
these arguments proved to be false.

There were so many
misstatements of facts in these four submissions that I felt obliged to submit
a detailed rebuttal for each. (Copies of my submissions are available to anyone
interested.) In the case of ATSC and Philips, I attempted to get knowledgeable
persons known to me in those organizations to deal with my objections, but no
response was ever forthcoming.

In this short
piece, there is insufficient space for presenting the detailed rebuttal of these
erroneous statements. Briefly, 1 and 2 relate to the 2 million/1 million issue,
and are dealt with below. As for 3, I receivers are slightly cheaper than P
receivers, but a P-to-I converter can be built into an MPEG decoder at nearly
zero cost for use with P broadcasts. (Note that an MPEG decoder for 1080I needs
more than twice the memory as a decoder for 720P, so costs more, not less.)
As for 4 and 6, P broadcasting does require the upconversion from I to P at
the studio for archival NTSC. This costs almost nothing for most of the material,
which originated with 24-fps film. In any event, the cost of the I-to-P converter
at the sending end is entirely negligible compared with the cost of converting
to any kind of digital transmission. Item 5 disappeared with the development
of an excellent 720P camera by Polaroid in 1996 and the demonstration of a 720P
camera by Panasonic at the recent NAB convention. Many who saw the Panasonic
720 P demonstration said that the pictures were the best TV they had ever seen.

An interesting
and highly relevant development occurred beginning in 1994 when various laboratories
began looking into the relative compressibility of P and I video. With a P and
an I signal having the same number of lines/frame and the same field rate and
horizontal resolution (e.g., 480x720 P 60 frames/sec and 480x720 I 30 fps) the
P signal has twice the analog bandwidth as the I signal. However, because of
the much higher statistical correlation and lower level of aliasing present
in the P signal, both can be MPEG-compressed to the same digital data rate with
about the same subjective quality. Results like these have been reported by
Bell Labs, NHK, RAI, and Project Race of the EU. Thus, there is no data-rate
penalty for using P rather than I, and there are many advantages.

As a result of
all these considerations, my conclusion is that there is no disadvantage,
monetary, quality-wise, or convenience-wise to any domestic stakeholder from
using P rather than I transmission. It is true that manufacturers who have
made an unwise investment in this obsolete technology would suffer a temporary
setback. However, should the US broadcasting industry choose progressive transmission,
I am also sure that these same companies will produce the necessary P products
in short order.

Interlace and
Resolution

All current analog
TV systems use interlace, in which the odd lines are transmitted on one field
and the even lines on the next. This was originally done in order to double the
large-area flicker rate at a given bandwidth, but it can just as well be thought
of as a means to double the vertical resolution by offsetting successive fields
by one-half the line spacing. The hope was to achieve the doubled flicker rate
and the doubled vertical resolution at the same time. However, there is no free
lunch. The only circumstances under which this can be done is when the two successive
fields are taken from the same (still) frame and printed on film. When there is
motion and/or when the integration of the two fields is done in the eye, the scheme
does not work as hoped for. This has been known for many years. A paper by E.F.Brown
of Bell Labs (BSTJ 46,1,1967 pp 199-232) showed that the degree of resolution-enhancement
actually attained depended on the screen brightness; at normal brightness, it
is only 10%, not 100%! Thus interlace never really worked, even in analog systems;
it only seemed to.

Interlace produces
many artifacts in the image. The most common is interline flicker, which is
caused when adjacent lines in the frame (which are transmitted and displayed
one field-time apart) are not identical. In other words, whenever there is good
vertical resolution, there is interline flicker. This is the reason why interlace
has been abandoned in computer monitors; computer video has full vertical resolution.
Camera video, however, always has reduced vertical resolution. In tube cameras,
this comes about automatically, since the physics of the camera causes the target
to be discharged completely every field, thus averaging (blurring together)
successive lines of each frame. In CCD cameras, this is done deliberately by
discharging two lines of photosites at once. If this were not done, the interline
flicker would make the image unwatchable. Those whose experience is limited
to conventional TV practice will not have seen this problem to its full extent.

The maximum possible
degree of interline flicker can be imagined by thinking of an NTSC image in
which the even lines are white and the odd lines are black. While this is certainly
an unusual picture, it is NTSC-legal. Such a display would flicker at 30 hz,
and the flicker would be perceived at any distance. It is the extent to which
adjacent lines differ (i.e., the extent to which they represent vertical detail)
that produces the flicker. For years, we had a demo of this effect in my lab
at MIT. None of the hundreds of TV professionals who came through had ever seen
this before and none had imagined that the effect was so large.

The necessity
of reducing the vertical resolution to avoid totally unacceptable interline
flicker means that the nominal resolution of interlaced systems is not the resolution
actually achieved in practice. The vertical resolution actually achieved is
usually not significantly higher than the number of lines per field,
not the number of lines per frame. For example, I have never seen an
1125 demonstration in which the limiting vertical resolution was more than 700
lines.

At one time,
the CBS laboratory in Stamford, Conn., had an NHK system that had been modified
so that it could quickly be switched between 1125 lines interlaced and 562 lines
progressive. When switched to P, there was no visible reduction in vertical
resolution. The only effect was to make the line structure somewhat more visible.

Other artifacts
of interlace include image break-up when the camera is panned vertically.
When
the vertical motion is one line/field, then half the display lines disappear.
Transcoding is also made more difficult (This is the reason why PAL<>NTSC
transcoding is imperfect even after decades of trying.)

There is general
agreement that P provides better images than I, so lip service is paid to an
eventual migration from I to P. The I advocates, however, insist that it is
too early to do so, for the various reasons mentioned above. This latest argument,
here shown to be entirely without merit, is simply the most recent attempt to
promote the use of existing interlaced production equipment at least for the
initiation of digital broadcasting.

2 Million vs
1 Million

As shown above, an
interlaced signal with 1080 lines per frame has an actual vertical resolution
barely half that, while a progressive signal of 720 lines per frame has an actual
vertical resolution of nearly 720. In the ATTC tests mentioned above, the objectively
measured vertical resolution of 720P was higher than that of 1080I. As
for the horizontal resolution, 1920 is indeed much higher than 1280, and if it
had been achieved, one would expect that the perceived sharpness of the I image
would have been higher than that of the P image. However, that was not the case.
The subjective sharpness as measured by ATEL was about the same. (The subject
matter was not specifically selected to illustrate interlace artifacts.) It is
clear that the 1080I image did not resolve 1920 pixels horizontally. In all likelihood,
this was caused by the camera itself or its filtering. It should be noted that,
with a 30 mhz bandwidth as used in the tests, the resolution is limited to about
1550 horizontal pixels.

Additional data
on this issue has emerged in Japan and at the recent NAB show. In Japan, the
Association of Radio Industries and Businesses (ARIB) has already changed 1080x1920
to 1080x1440 because the higher resolution causes coding artifacts (blocking)
that can be reduced or eliminated, depending on the scene, by some reduction
in horizontal resolution. There were also reports from NAB of blocking artifacts
in 1080I coded material, no doubt from the same cause. Last December, Sony requested
ATSC to change the 1080x1920 format to 1080x1440. On the other hand, there were
no reports of compression artifacts with 720P at NAB.

In summary, the
nominal resolution of 1080x1920 is not achieved in practice. The 1080I format
does not have higher resolution than the 720P format, and it has all the well
known interlace artifacts. There is no quality advantage in using 1080I,
and there are no valid reasons not to use progressive scan.

Conclusion

The idea that 1080I
has higher resolution than 720P has been shown to be false. The resolution actually
achieved in the interlaced system is far below the nominal 1080x1920. The reduction
in vertical resolution is due to the need to lessen the interline flicker that
would otherwise be present. The reduction in horizontal resolution is partly a
camera problem and partly a limitation of the MPEG compression system. These limitations
are inherent; they cannot be removed within the given transmission data rate.
There was a time when these matters were not fully understood, but that time is
long past. There is now a mountain of evidence that shows that there is no advantage
whatsoever to using interlace in digital TV broadcasting except to the manufacturers
of interlaced production equipment. The fact that some interlace advocates are
still pushing this obsolete technology shows that their viewpoint cannot be based
on facts, but is almost surely due only to their last-ditch attempt to make the
already developed 1125-line production equipment the appropriate equipment to
use as HDTV broadcasting is initiated.

Glossary

ATTC Advanced Television
Testing Center

ATEL Advanced
Television Evaluation Laboratory of the Canadian Dept. of Communications

NTSC National
Television Systems Committee. The current analog TV system used in the US

PAL Phase alternation
by line. The current analog TV system used in most 50-hz countries

MUSE The analog
compression system used for transmitting NHK signals by satellite

SMPTE Society
of Motion Picture and Television Engineers

ANSI American
National Standards Institute

EBU European
Broadcasting Union

NHK Japan Broadcasting
System. Also the 1125-line interlaced system developed by NHK

ACTV Advanced
Compatible Television. An analog system compatible with NTSC, developed at the
Sarnoff Laboratories.

ACATS Advisory
Committee on Advanced TV Systems.

ATSC Advanced
Television System Committee

MPEG. Motion Picture
Experts Group. Also the digital compression system developed by MPEG.

ARIB Association
of Radio Industries and Businesses of Japan

NAB National Association
of Broadcasters

RAI Italian Broadcasting
System

EU European Union

N.B. Numbers such as 720x1280 refer to the structure of the visible television
frame. Analog systems such as NTSC have a larger total number of lines (525) as
compared with the 480 lines of the visible frame. The original NHK system had
1125 total lines, of which 1035 formed the visible image.

N.B. This note represents
the personal opinion of the author, who has no financial interest in the outcome
of the matters discussed herein.